You should talk to the ROTC Battalion at the college you plan to attend before you sign up for anything. They will be able to give you all the information regarding the SMP program. If you enlist you would then be a SMP cadet. if the battalion has slots available you could contract as early as the start of your sophomore year.
Don't just talk to your local recruiter, go talk with the ROTC Battalion first, they can help you through the process and make sure your on the right track.
Although there are ways you can skip your first 2 years of ROTC since you will have completed both BCT and AIT it's really not a good idea. FireFighter is right, it's best to complete all 4 years of ROTC.
Just as a side note, you can join ROTC, do the SMP program, and drill with a NG unit and not be required to attend either BCT or AIT, or you can just go to BCT and not go to AIT. This is why you should be talking with ROTC right now and not the local recruiter, you need to understand all your options and all the risks involved before you just go and enlist.
